The present invention relates to radiation dosimetry, and more particularly to methods and devices for automating radiation dose calibrations associated with radiotherapy.
An important use of radiotherapy is the destruction of tumor cells. In the case of ionizing radiation, tumor destruction depends on the “absorbed dose” or the amount of energy deposited within a tissue mass. Radiation physicists normally express the absorbed dose in cGy units or centigray. One cGy equals 0.01 J/kg.
Radiation dosimetry generally describes methods to measure or predict the absorbed dose in various tissues of a patient undergoing radiotherapy. Accuracy in predicting and measuring absorbed dose is key to effective treatment and prevention of complications due to over or under exposure to radiation. Many methods exist for measuring and predicting absorbed dose, but most rely on developing a calibration—a curve or a lookup table—that relates the response of a detection medium to absorbed dose. Useful detection media include radiation-sensitive films and three-dimensional gels (e.g., ‘BANG’ and ‘BANANA’ gels) which darken or change color upon exposure to radiation. Other useful detection media include electronic portal-imaging devices and amorphous silicon detector arrays, which generate a signal in response to radiation exposure.
In order to develop a calibration curve or lookup table, discrete portions of the detection medium are exposed to different and known amounts of radiation using a linear accelerator or similar apparatus. Typically, about twelve, but often as many as twenty-five different radiation dose levels are measured in order to generate a calibration curve or look-up table. Generally, the accuracy of the calibration increases as the number of measured radiation dose levels increases. However, measuring separate radiation dose levels is a labor intensive and time consuming process, which can be demonstrated by examining a calibration process for radiation film dosimetry.
As shown in FIG. 1A and FIG. 1B, a linear accelerator 100 is used to expose different areas of a radiographic calibration film 102 to ionizing radiation 104. The film 102 is typically sandwiched between layers 106 of material that mimic the response of human tissue to the ionizing radiation 104. A shield 108, which is made of a dense material such as lead, is interposed between the film 102 and the linear accelerator 100. The shield 108 has a fixed aperture 110, which only permits ionizing radiation 104 to reach an area of the film 102 that is aligned with the aperture 110. During calibration, the area of the film 102 that is aligned with the aperture 110 is exposed to a known and unique dose of ionizing radiation 104, while other areas of the film 102 are masked by the shield 108. The absorbed dose can be obtained from ion chamber measurements, Fricke dosimetry, calorimetry, or other absolute dosimetry methods. Because of the high radiation levels involved, the physicist must leave the room and close shielding doors during each exposure of the film 102. After exposing the film 102 to radiation, the physicist reenters the room and moves the film 102 to align a previously unexposed area of the film 102 with the aperture 110. The physicist then leaves the room, secures the shielding doors, and sets the linear accelerator 100 controls to deliver the next dose level. This process is repeated for each dose level in the calibration sequence.
FIG. 2 shows three radiographic calibration films 102 that have been exposed to different radiation dose levels during a calibration sequence. Each of the films 102 have discrete areas 120 that have been exposed to different radiation dose levels, which range from 0 cGy to 220 cGy. Normally, the radiation dose levels of the calibration films 102 are obtained throughout a range of dose levels that are expected during radiation therapy, and generally range from 0 cGy to as much as 6,000 cGy. The step sizes between successive calibration dose levels can vary and depend on the dynamic range of the detection medium used. Ordinarily, the calibration films 102 are scanned with a film digitizer, which converts each of the films to an array of pixels having values representing the optical density at each point on a particular calibration film 102.
FIG. 3 shows a sample calibration or H&D curve 140 obtained from the calibration films 102 shown in FIG. 2. Usually, specialized software averages the optical density over the discrete areas 120 of the calibration films 102, and generates a calibration curve or look-up table based on known values of the radiation dose levels and the measured optical density. Armed with the H&D curve 140 (or other calibration), the radiation physicist can quantify beam characteristics of the linear accelerator through subsequent exposure, development, and optical density measurements of radiographic films. For example, as part of a treatment plan or quality assurance procedure, the radiation physicist can use film dosimetry to generate depth dose profiles, isodose and isodensity contours, and cross section profiles. In addition, the physicist can use film dosimetry to perform flatness and symmetry analyses, and to carry out field width calculations, among others. Usually, the physicist uses computer software that automatically calculates and displays beam characteristics from scanned and digitized radiographic films. Useful software for generating the H&D curve and for analyzing radiotherapy beam characteristics includes RIT113 FILM DOSIMETRY SYSTEM, which is available from Radiological Imaging Technology.
Calibration procedures, such as the method described above for film dosimetry, have several disadvantages. First, regardless of the detection media used, the methods require a large number of labor- and time-intensive steps to expose the requisite dose levels needed to generate the calibration. Second, care must be taken to ensure that in each calibration step a previously unexposed area of the detection media is used. If the dose areas overlap, the calibration data can be meaningless. Third, because of the relatively large number of films that must be exposed in film dosimetry, short-term drift in radiation response from one film to the next can occur because of changes in film processor chemistry and temperature.
The present invention overcomes, or at least mitigates, one or more of the problems described above.